The 16-year-old, who was arrested Saturday, also is charged with dumping a hazardous material, as well as conspiracy to commit dumping, cruelty to children, theft and receiving stolen property. It was the first time the youth has been arrested, a court social services officer told the judge.

Surrounded by his family, the 16-year-old appeared in D.C. Superior Court before Magistrate Judge Milton C. Lee in the first court hearing in the case.

Even in small amounts, mercury, a metallic element used in some thermometers, can pose a health risk if left exposed in a confined space, according to the Environmental Protection Agency.

The brief hearing last night did not delve into the details of the charges, but the conspiracy, theft and stolen property charges all suggest that prosecutors believe the boy's actions were more than a spontaneous act.

The incident was recorded by a surveillance camera at Cardozo, school officials said, and the footage helped lead police to the 16-year-old, who spoke in court only to state his name, age and date of birth.

Juvenile hearings are closed to the public, but the media was permitted to attend yesterday's proceedings on the condition that neither the youth nor his family be identified.

The contamination of the Columbia Heights school Wednesday forced administrators to evacuate Cardozo and then close it for cleaning. On Sunday, the D.C. Department of Health declared the school safe to reopen, but that was delayed by the snow-related closure yesterday of D.C. public schools.

The 16-year-old has said that he found the mercury in the school, according to authorities, and that has forced administrators to reevaluate claims that all mercury was removed from the school after a far more serious mercury spill at Ballou Senior High School in fall 2003.

In that incident, a student stole the substance from a school lab and gave it to his friends, who spread it throughout the building. The school was closed a month for cleanup.

Officials said they are examining records from all the high schools from late 2003, when the school system oversaw the removal of mercury from the schools' labs.

"In light of what the young man said, we're double-checking," said Thomas Brady, the school system's chief of business operations. He added that the schools' security department will report back to him this week. "We're confident there wasn't any [mercury in the Cardozo lab], but we want to verify that."

Cardozo Principal Reginald Ballard Jr. said yesterday that the school system in 2003 hired a consultant who oversaw the removal of mercury from the school.

"He identified chemicals that were old and those that were banned and moved them into metal closets," Ballard said, adding that even mercury thermometers were removed.

Ballard added that the removal of the mercury "was not a function of the school. It was a function of the consultant and the DCPS."

The next hearing in the case, scheduled for later this month, may shed more light on how the government believes the mercury ended up in the hands of the student.

Assistant D.C. Attorney General James H. Vricos did not object to the youth's release into the custody of his parents, but he asked for him to be placed under intensive supervision. But after speaking to the boy's family and listening to the arguments of the boy's attorney, Gustavo Gutierrez of the D.C. Public Defender Service, Lee agreed to release him.

"He's coming home," his mother said afterward. "That's all that matters."